I switched from the default driver to fglrx recently so a certain app would stop crashing. After a while I got my dual monitor system to work, except the right monitor (Acer) shows the whole desktop shunken about 90%. There's an unused margin of an inch or so all around.
Both monitors are 23", identical for functional purposes but different brands. Both are displaying at 1920x1080 and show the entire desktop. Left monitor, which shows the whole desktop raster filling the physical LCD area, is an AOC plugged in by DVI. The right monitor is an Acer plugged in by HDMI. It's not possible right now to wrestle these around to see the labels on back; exact models unknown. Both are plugged into an ATI Radeon HD 5450.
Catalyst Control Center says it's version 2.13, Catalyst version 11.8.
Using XUbuntu 11.10, maybe a week or two not up to date. Using XFCE, but tried KDE and Unity to see if that mattered. Nope, except Unity made the right monitor all back except for the cursor.